Flash memory is a nonvolatile memory that was developed from electrically erasable programmable read-only memory (EEPROM). NAND flash memory and NOR flash memory are available. NAND flash memory can be written or read in blocks or pages. Originally, each flash memory cell had one bit per cell. Now, flash memory is available in single level cell (SLC) and multilevel cell (MLC) types. The single level cell type of flash memory has one bit per cell (as on the original flash memory), and the multilevel cell flash memory types have more than one bit per cell. Generally, multilevel cell flash memory operates with each cell capable of multiple ranges of electrons trapped on a floating gate, multiple programming voltages, and multiple threshold voltages applied to a controlling gate within the flash memory cell. The number of bits per cell is a function of the number of ranges of programming voltages and threshold voltages, and corresponding numbers of electrons on the floating gate. One type of multilevel cell flash memory is available is referred to as triple level cell (TLC) flash memory, which has three bits per cell. Further types of multilevel cell flash memory may be developed.
Flash memory is attractive as a storage media due to the access speeds. The familiar USB (universal serial bus) flash drive is an example of portable storage that makes use of flash memory. Solid-state drives, such as a USB flash drive, usually use the NAND type of flash memory. Trade-offs in cost per bit, bit density per unit of area in an integrated circuit, error rates, long-term storage reliability, read disturb rates, read wear rates and write wear rates, among other considerations, affect the choice of which type of flash memory to use in a given application. Generally, a single type of flash memory is selected for use in a system, e.g., a thumb drive, a solid-state drive, or a combination hard drive and solid-state storage, and the system then has the advantages and disadvantages of that type of flash memory. It is within this context that the embodiments arise.